FIG. 9 shows a conventional steering mechanism of a vehicle. In FIG. 9, reference symbols a and b denote telescopic shafts. In the telescopic shaft a, a male shaft and a female shaft are in spline fitting. Such a telescopic shaft a is required to have a function of absorbing displacement in the axial direction which is generated during running of the vehicle and of preventing such displacement or vibration from being propagated onto a steering wheel. Such a function is usually required when the vehicle body is in a sub-frame structure in which a portion c for fixing an upper part of the steering mechanism and a frame e to which a steering rack d is fixed are separately provided and both members are fastened and fixed through an elastic member f such as rubber. There is also another case where a telescopic function is required for an operation, when fastening a steering shaft joint g to a pinion shaft h, to temporarily contract the telescopic shaft to then fit and fasten it to the pinion shaft h. Further, the telescopic shaft b in an upper part of the steering mechanism also comprises a male shaft and a female shaft being in spline fitting. This telescopic shaft b is required to have the ability of extending and contracting in the axial direction since it is required to have the function of moving the steering wheel i to adjust the position thereof in order to obtain an optimal position for the driver for driving the vehicle. In all the cases described above, the telescopic shaft is required to reduce backlash noise in a spline portion, to reduce the backlash feel on the steering wheel, and to reduce a sliding resistance during a sliding movement in the axial direction.
On that account, in FIGS. 1 through 6 of Japanese Patent Application Laid-Open No. 2001-50293 (FIGS. 1 through 6 and FIG. 12), three sets of spherical bodies or balls serving as torque transmitting members are fitted between the three sets of axial grooves formed on the outer peripheral surface of a male shaft and on the inner peripheral surface of a female shaft.
With this arrangement, backlash between the male shaft and the female shaft can be prevented when torque is not transmitted, and the male shaft and the female shaft can slide in the axial direction with a stable sliding load without backlash. On the other hand, the male shaft and the female shaft are capable of preventing backlash in the direction of rotation thereof so as to transmit torque in a state of high rigidity when the torque is transmitted.
In FIG. 12 of Japanese Patent Application Laid-Open No. 2001-50293 (FIGS. 1 through 6 and FIG. 12), while the three sets of spherical bodies or balls serving as the torque transmitting members are interposed between the three sets of axial grooves formed on the outer peripheral surface of the male shaft and on the inner peripheral surface of the female shaft, another three axial grooves are formed at regular intervals between the three axial grooves of the female shaft in the circumferential direction and furthermore, three projections are formed at regular intervals between the axial grooves of the male shaft in the circumferential direction. These three projections are fitted in the latter three axial grooves separately provided. However, there is allowed a predetermined gap, that is, play between the latter three axial grooves and the three projections in the radial direction.
With this arrangement, even when the spherical bodies or balls serving as the torque transmitting members are broken, the three projections are fitted in the latter axial grooves, whereby the male shaft and the female shaft can transmit torque and can discharge a fail safe function.
However, in Japanese Patent Application Laid-Open No. 2001-50293 (FIGS. 1 through 6 and FIG. 12), the three sets of the spherical bodies or balls serving as the torque transmitting members, the three axial grooves separately provided and the three projections are provided on the same cross section (the same surface in the circumferential direction), so that the radial dimensions of the male shaft and the female shaft become great and the sizes thereof can not be reduced.
Also, in Japanese Patent Application Laid-Open No. 2001-50293 (FIGS. 1 through 6 and FIG. 12), since the torque transmitting members consist only of the spherical bodies or balls, there is required the sufficient number of the spherical bodies or balls for enduring the surface pressure which is applied onto the spherical bodies or balls. As a result, the axial grooves are elongated and the axial dimensions of the male shaft and the female shaft become great, so that the telescopic shaft can not be formed compact.